It looks similar in structure to JSON:

{
    "attr": {
        "size": "62091",
        "filename": "qBuUP9-OTfuzibt6PQX4-g.jpg",
        ...
    };
    "key": "Wa4AJWFldqRZjBozponbSLRZ",
     ...
}

So, it might be some JSON meta language. I just find it weird that it seems to contain all data, so you wouldn’t use this for validating or templating JSON.

But ultimately, it also means with a handful of regex replacements, you could turn this particular file into JSON. Might make building your own parser almost trivial…